Giving a Reason

As, since, because and for are all conjunctions that connect the result of something with its reason. While they have the same function, they are used in different manner. They should be used carefully to give clear information.
As and since are used when the reason is already known. They can also be used when the result is more important in the sentence. Study the following examples:
As he was sick, he was unable to take the test.
Since she did not come on time, her friends went to the party without her.
As and since are used in the same manner in both sentences. They are both found in the dependent clause of the sentence. A comma is used when attaching the subordinate clause to the main clause.
In speaking and in writing because is more commonly used. It introduces the reason which is not yet known to the reader or listener. The reason is the most important part of the sentence. as in the following:
Eric bought a new car because his old car was beyond repair.
Because food is very important to our health, skipping meals should be avoided.

When using because at the beginning of a sentence, a comma is necessary.
For, like because introduces new information. For however, shows a less direct relationship between two ideas. Unlike because, since and as which are all subordinate conjunctions , for is a coordinating conjunction. A comma is placed before for as in these sentences:
Somebody must have eaten my apple, for I don't see it on the table where I left it.
Corine fell asleep, for she was so tired.